1. Field of the Invention
The present invention relates to a screw rotor mechanism for screw compressors, and more particularly a pair of male and female screw rotors for use in dry-sealed screw compressors in which the male and female rotors are rotated by a synchronism means such as timing gears in a synchronized timing and mechanically spaced relation.
2. Description of the Prior Art
The screw rotors of this dry-sealed type are necessarily designed to maintain a gap large enough to avoid an objectionable interference therebetween which will otherwise occur mainly due to a thermal expansion thereof during its operation, particularly since the dry-sealed rotors commonly easily increase its temperature, namely up to more than 200.degree. C.
Conventionally the dry-sealed male and female rotors are mounted within a casing of the compressor in consideration of an inter-axis distance "CD" between axes of the two rotors. Therefore the axes thereof are positioned so as to be sufficiently spaced apart from each other to thereby prevent a mechanical interference between the rotors which would otherwise occur due to the thermal expansion of the rotors during operation. An expansion amount .DELTA.L of the two rotors is determined by EQU .DELTA.L=CD.times..beta..times..DELTA.T
wherein .beta. is a linear expansion factor, and .DELTA.T is an expected increase of temperature.
The axes of the two rotors are positioned such that a tooth tip circle of the male rotor and a root circle of the female rotor are spaced apart from each other by a distance (i.e., inter-tooth distance) determined by sum of the foregoing expansion amount and a minimum safety gap enabling operation.
In view of a compression efficiency, the inter-tooth distance is important in that an objectionable leakage of the compressed air at a suction side of the compressor can be decreased by minimizing the inter-tooth distance at a seal point between the rotors. The inter-tooth distance is determined by the foregoing inter-axis distance between the rotor axes and further by expanded tooth profiles resulted from their thermal expansion.
However, customarily such expanded tooth profiles have not been considered as a designing point for providing a minimized inter-tooth distance or gap. As a result, conventional attempts made to design an improved rotor tooth profiles without considering the thermal effect such as a thermal expansion or deformation were unsuccessful because the thus designed rotor tooth profiles inevitably deformed at an increased temperature in operation, thus deteriorating the compression efficiency.
Recently, a number of improved rotor tooth profiles have been proposed in view of the thermal expansion, as disclosed, for instance, in Japanese Patent Laid-Open Publications Nos. 57-159989, 59-37291 and 59-58189. As those documents indicate, thermal expansions occur non-uniformly or unevenly throughout respective peripheries of the male and female rotor teeth. It is well understood with reference to FIGS. 4 and 5 of the accompanying drawings in which normal-temperature tooth profiles of the male and female rotors M, F (indicated by solid line) are remarkedly different from increased-temperature tooth profiles thereof (indicated by dot-and-dash line). It can be generally observed in FIGS. 3A and 3B that the closer a tooth profile point is located to the rotor axis, the less expansion occurs. More precisely, the expansion amount at each tooth profile point can be calculated by a mathematical method such as infinite element method.
In the screw compressors of such dry-sealed type, the seal coating also affects the inter-tooth gap, in which a seal material such as a seal coating of molybdenum disulfide is applied to the peripheral surface of the rotor tooth as a lubricant for preventing an objectionable seizure caused by a direct interengagement by and between the teeth of the male and female rotors. Such seal coating applied to the rotor teeth of the foregoing prior rotors is uniform in thickness throughout the entire peripheral surfaces of the rotor teeth because relating to the restrictions of coating technique.
However, it has been found that since the thermal expansion amount varies at the tooth profile points, thus uniformly applied seal coating creates a drawback in that some tooth profile points of the male and female rotors, under the expanded condition, provide an inter-tooth gap smaller than the thickness of the seal coating applied thereto. When the seal coating thickness is greater than the inter-tooth gap as described hereinabove, the pair of male and female rotors cannot be mounted in proper intermeshing relation within the compressor casing.